1. Field of the Invention
The present invention relates to a heated extended nip press apparatus and a method for obtaining optimum web properties while inhibiting delamination of the pressed web.
2. Information Disclosure Statement
Extended nip press arrangements have enabled the removal of far greater quantities of water from the formed web when compared with more conventional roll presses which include at least a pair of cooperating rolls defining therebetween a press nip.
More particularly, such extended nip presses have reduced the costs involved in the subsequent drying of the pressed web in a dryer section.
An extended nip press essentially includes a backing roll and a press shoe having a concave surface which cooperates with the backing roll for defining therebetween an extended nip. An endless looped blanket moves contiguously with a felt and the formed web through the extended nip with the interface between the blanket and the shoe being lubricated. The felt acts as a carrier for the weak wet web and collects water therefrom. Accordingly, by the application of pressure for an increased period of time, the increased residence time not only enables the removal of more water from the formed web but also in certain applications improves the properties of the resultant web.
More recently, as disclosed in U.S. Pat. No. 4,738,752 to Busker et al, the extended nip press arrangement has been enhanced by the application of heat to the backing roll, such heat being applied to the backing roll typically by means of an induction heater.
Evidently, the application of induction heating reduces the viscosity of the water within the formed web thereby facilitating the removal of more water from the web. Additionally, the application of thermal energy causes the generation of steam within the extended nip, such that the steam within the web forces water in the liquid phase out of the web thereby further enhancing the water removing capabilities of the extended nip press.
However, in view of the rapid generation of steam within the extended nip, there existed a certain tendency of the web to delaminate on exiting from the extended nip. Accordingly, various temperatures were used with different residence times and pressures in an attempt to minimize such tendency to delaminate.
During the course of such experimentation, a discovery was made that by gradually decreasing the pressure in a machine direction towards the trailing edge of the shoe, rapid flashing of steam from the emerging pressed web was avoided. Rapid flashing of steam from the web was recognized as being the main cause of the delamination problem.
By selectively changing the pressure applied on the blanket along a machine direction, not only was the problem of delamination alleviated but unexpectedly the properties of the resultant pressed web were enhanced.
Therefore, it is a primary object of the present invention to provide a heated extended nip press apparatus which overcomes the aforementioned problems associated with the prior proposals and which makes a considerable contribution to the art of high temperature pressing.
Another object of the present invention is the provision of a press means which includes means for selectively changing the pressure applied on a blanket along a machine direction relative to a further pressure applied for moving the press means relative to the backing roll such that optimum web properties are obtained and delamination of the pressed web is inhibited.
Another object of the present invention is the provision of a heated extended nip press apparatus having an induction heater disposed closely adjacent to the backing roll for heating the backing roll thereby increasing the amount of water removed from the formed web during passage of the web through the extended nip.
Another object of the present invention is the provision of a heated extended nip press apparatus which includes a hydrodynamic shoe defining therein a plurality of cross-machine directional recesses. The recesses selectively cooperate with a piston so that the shoe is permitted to pivot about the piston, the shoe being selectively positioned in a machine direction relative to the piston to optimize the web properties and to inhibit delamination of the pressed web.
Another object of the present invention is the provision of a heated extended nip press apparatus in which the press means includes a rod rigidly secured to a piston and disposed between the piston and the shoe. The rod extends in a cross-machine direction such that the rod pivotally supports and is selectively disposed within one of the plurality of recesses.
Another object of the present invention is the provision of a heated extended nip press apparatus which includes a hydrostatic shoe defining a plurality of pockets disposed in a machine direction with each of the pockets being selectively connected to a source of hydraulic pressure such that the blanket is hydrostatically supported by the shoe and so that optimum web properties are obtained and delamination of the pressed web is inhibited.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.